The deposition of particles on articles or substrates may employ various processes including hot spray processes that use combustion or plasma as the heat source. The hot spray of materials or particles on substrates, such as components in a gas turbine for example, may produce multiple surface layers on the substrate having properties that may gradually change and complement the properties of the substrate. The deposited surface layer is intended to protect the substrate during its service against aggressive and hostile environmental attack and extend the component's service life and performance.
Known methods of hot spraying materials may use a continuous heat source to partially and/or fully melt the aggregate of material to be deposited. The aggregate of material may be in a suitable form of feedstock such as non-agglomerated powder, cast rod, wrought or pellet-encapsulated wire or a solution with chemical pre-cursors that is continuously delivered into the heat source. The form of feedstock for metallic and/or non-metallic materials may include powders of uniform size, powders with a known size distribution and/or powders encapsulated in fine tubes such as the commercially available Sultzer-Metco nickel-aluminum AMDRY 404NS feedstock.
Spray torches or guns known in the art may use combustion and plasma gases as heat sources for hot spray. The heat source may be enclosed in a compact container with inlets and outlets for the reactants used for combustion or the plasma gases used for plasma generation and for cooling fluid. The container may also include a nozzle for spraying and a method of introducing the depositing materials into the combustion products or plasma beam. Combustion based hot spray processes, or thermal spray processes, may be performed at atmospheric air pressure. When an oxy-fuel is used in the combustion process, the spray process is called High Velocity Oxy-Fuel (HVOF) process. Low-pressure plasma spray (“LPPS”) or vacuum plasma spray (“VPS”) processes are known as well as an air plasma spray (“APS”) process. In thermal spray and plasma spray processes using powder feedstock, the depositing material may be introduced into a hot plume either concentrically or downstream at an angle to the plume axis by using a carrier gas. The high velocity combustion or plasma gases propel particles entrained within a spray beam or jet onto the surface of the substrate to be coated. These particles may consist of fully and/or partially molten particles, as well as un-melted hot particles. Individual particles within the spray jet may have a range of properties such as temperature, velocity and diameter. Commercially available in-flight particle analyzers, such as the DPV 2000 manufactured by TECNAR, allow for quantitatively measuring these and other in-flight particle properties.
Known suppliers of equipment used for hot spraying may provide ranges for each operating parameter of the equipment and projections regarding a resultant coating's quality and structure. This may be done by trial and error, design of experiments or other iterative approaches by using a flat plate as a target to arrive at such ranges of operating parametric values that are provided to the end user of the spray equipment. For example, an operator may select a set of operating parameters and spray a test coating on a substrate. A metallurgical analysis of the test coating may then be performed to determine whether the coating is within specified coating quality and structure limits. If the test coating is not within these limits then the operator may repeat the process not only changing the gun and powder feed operating parameters within recommended ranges, but also varying the spray gun speed, its spray or “standoff” distance from the plate and lateral stepwise displacement, for example, until the test coating specifications are acceptable.
Once the operating parameters are optimized for a specified coating, it is expected to reproduce the specified coating quality and structure on subsequent coating runs, article-to-article and batch-to-batch. However, reproducing the specified coating on subsequent coating runs often proves to be difficult even though the same optimized operating parameters are used on restart of the hot spray system. This may be due to the non-reproducibility of the expected in-flight particle properties on restart. This may result in a wide variation in coating quality and structure from one location to another on a coated substrate, from one coated substrate to another and from batch-to-batch of coated substrates. These results may cause the rejection of non-conforming substrates, stripping the non-conforming coating and re-coating, or relaxing the coating specifications to reduce the rejection rate. Each of these results increases the cost of production and/or compromises the performance of the coated article.